Alloy for elevated temperatures



United States Patent 3,166,406 ALLOY F011 ELEVATED TEMPERATURES Stephen Fioreen, Westiield, and Raymond F. Decker, Fanwood, N.J., assignors to The International Niche! Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 3, 1962, Ser. No. 214,475 6 Claims. (6!. 75-124) The present invention relates to ferrous-base alloys and, more particularly, to strong, ductile weldable ferrousbase alloys for use at temperatures up to about 1000" F. and higher. t

It is well known that certain ferrous-base alloys, that is, alloys containing iron as their major constituent, can be produced with a martensitic matrix which can, by means of tempering and/or hardening heat treatments, provide in those alloys advantageous combinations of high strength, wear resistance, hardness, etc., at room temperature or below. Particularly advantageous age-hardenable martensitic ferrous-base alloys have been disclosed in the Bieber US. patent application Serial No. 839,296, filed on September 11, 1959 (now US. Patent No. 3,093,518), and in the Decker et al. US. patent application Serial No. 80,381, filed on Ianaury 3, 1961 (now US. Patent No. 3,093,519). The prior Bieber US. patent application Serial No. 839,296 disclosed iron-base alloys containing, in percent by weight, about 18% to about 30% nickel, small interrelated amounts of carbon and oolumbium (niobium) and amounts of titanium and/ or aluminumin the aggregate of at least about 1.5%. These Bieber alloys can be subjected to an age hardening heat treatment while in the martensitic condition to provide hitherto unattainable combinations of strength and ductility at room temperature and lower. The aforementioned Decker et al. US. application was concerned with ferrousbase alloys containing about 10% to about 28% nickel together with interrelated amounts of cobalt and molybdenum which could also be age hardened while in the martensitic condition. The alloys of both Bieber and Decker et al. have been widely publicized in metallurgical circles and are currently in the process of being tested for acceptance in many applications where strength coupled with toughness at ordinary temperatures is of paramount consideration. These alloys, known as maraging steels, are characterized on the whole by ease of formability both by hot-working and cold-working processes, by case of weldability and by freedom from major distortion during hardening heat treatment.

Since the discovery of the remarkable combination of characteristics which can be provided by the aforementioned steels for applications at ordinary temperatures,

considerable effort has been expended in endeavoring to provide alloys having similar advantageous characteristics at elevated temperatures of the order of 800 F. to 1000 F. and higher. Such steels known prior to the present invention. more or less uniformly show a marked deterioration of strength when subjected to short time tensile tests and/ or stress rupture tests at 1000 F.- For example, at 1000 F known steels of the prior art containing about 18%. nickel, about 7.5% cobalt, about 4.8% molybdenum, about 0.4% titanium, about 0.1% aluminum and up to 0.03% carbon, exhibit yield strengths of less than about 130,000 pounds per square inch (p.s.i.). The obvious expedient of merely increasing the room temperature strength of the steels to compensate for the thermally induced strength loss was not eifective'in providing an alloy commercially usable for hot work dies and tooling and other components, parts, structures etc., subjected in use to elevated temperatures of the order of 1000 F. because steels modified in this manner exhibit a substantial loss of room temperature toughness. As is well known, an engineering material for elevated temperature use must usually exhibit good strength and toughness characteristics both at high temperature and ambient temperatures, particularly in the innumerable instances where service conditions include cyclic variations among high, intermediate and ambient temperaturm. An additional factor to be considered with regard to an alloy as a high temperature engineering material is the resistance of the alloy to corrosive attack by the atmosphere. Thus, to be accepted industrially, an alloy for high temperature use should resist oxidation and/ or scaling at least as well as materials in current competitive use. Although attempts were made to provide an alloy exhibiting a combination of all the mechanical and/ or physical and/ or chemical characteristics industrially required in an engineering material for use at high temperatures of the order of about 1000 F., none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that by means of a special correlation of alloying elements, a novel steel can be pro vided having an advantageous combination of elevated temperature and room temperature characteristics. It is an object of the present invention to provide a novel steel.

Another object of the invention is to provide a novel ferrous-base alloy for use in structures, components, parts, etc., employed at elevated temperatures up to about 1000" F. and higher.

The invention also contemplates providing a novel coldworkable steel.

his a further object of the invention to provide a novel age-hardened ferrous-base alloy for use at elevated tempertures up to about 1000 F. V

The invention further contemplates providing novel structures including components, parts, welded assemblies,

etc., subjected in use to elevated temperatures up to about 1000 F. and higher made of a substantially martensitic age-hardened ferrous-base alloy.

, Generally speaking, the present invention contemplates a novel ferrous-base alloy containing, in percent by weight, about 14% to about 16%'nickel, about 8.5% to about 9.5% cobalt, about 4.6% to about 5.2% molybdenum, about 0.1% to about 0.8% titanium, about 0.1% to about 1.2% aluminum, up to about,3% vanadium, up to about 0.03% carbon, up to about 0.2% manganese, up to about 0.2% silicon, up to about 0.01% boron, up to about 0.1% zirconium, up to about 0.05% calcium with the balance being essentially iron together with small amounts of impurities and incidental elements normally occurring in ferrous-base alloys and/or associated with said alloying elements. The ferrous-base alloy composition of the present invention is correlated so that the number resulting from the summation of five times the percentage of titanium, plus three times the percentage of aluminum plus the percentage of vanadium is at least equal to about 4.5 and is less than about 7. In other Words, H

ares toe Advanta-geously, alloys in accordance with thepresent invention contain about to about 0.9% aluminum and con, manganese, boron, zirconium, rare earth elements (from the addition of mischmetal), lithium, magnesium, uranium, calcium, etc. The alloy is then cast at temperatures of about 2800 F. into ingots. The ingots are subsequently hot worked at starting temperatures of about 2300 F. and finishing temperatures of about 1500 F., advantageously in more than one direction to facilitate homogenization of the microstructure. After hot working by rolling, forging or the like, the alloys are advantageously annealed at about 1800 F. for about 1 hour then cooled to room temperature in air or otherwise to effect a transformation of the alloy matrix and provide a structure comprising decomposition and/or transformation products of austenite. Following transformation, the alloy can be cold worked, machined, etc., and thereafter hardened by aging at a temperature of about 800 F. to

about 1000 F. for about 1 to about '10 hours. An agehardening treatment of about 3 hours at about 900 F. has been found to be satisfactory. More broadly speaking, the annealing heat treatment can consist of heating for about 1 to about hours at 1500 F. to about 1800 F. with the longer times being employed at temperaturesin the lower end of said temperature range. Examples .of wrought alloys in accordance with the present invention areset forth in Table I:

1 Iron includes small amounts of silicon, manganese, boron, zirconium and/or calcium within the limits of the ranges set forth hereinbefore together with residual andunavoidable amounts of impurities ane incidental elements.

Samples of the wrought alloys set forth in Table I were annealed at 1800 F. for 1 hour, air cooled to room temperature to effect a transformation and/ or decomposition of austenite, age hardened for about 3 hours at 900 F. and then air cooled to room temperature. Tensile characteristics of the thus heat treated samples were determined both at room temperature and at elevated temperatures. Yield strengths (Y.S.) in thousands of pounds per square inch (k.s.i.), ultimate tensile strengths (U.T.S), elongation (EL) and reductions in area (R.A.) for alloy No. 2 of Table I are set forth in Table II:

Table II shows that alloys in accordance with the present invention exhibit a high combination of strength and ductility over the wide temperature range of about room temperature (70 F.) to 1000 F. Thus, alloys in accordance with the present invention can be successfully employed in situations involving cyclical variations in temperature over a wide range.

Additional samples of alloys in accordance with the present invention, heat treated in like manner to the samples employed to obtain the data set forth in Table II, were tested in stress-rupture tests. The results of these stress-rupture tests are set forth in Table III:

Table III Test Stress Life, EL, R.A., Alloy No. 'Ienuaera (k.s.i.) Hours percent percent ture, F.

Table III shows that at elevated temperatures of about 800 F. to about 1000 F, alloys of the present invention exhibit extraordinaryresistance to creep under very high stress. This resistance to prolonged high stress at elevated temperatures coupled with the extraordinarily high combination of tensile characteristics over the temperature range of about 70 F. to about 1000 F. as shown in Table 11, indicates wide commercial utility for the alloys of the present invention as parts and components subjected in use to elevated temperatures and high stress for extended periods of time which are intermittently subjected to lower temperatures.

It is important to maintain the amounts of alloying elements within aforedisclosed ranges in accordance with the present invention in order to obtain an optimum combination of room temperature and elevated temperature characteristics. Varying the nickel content outside the range of about 14% to about 16% lowers the short time orlong time strength properties at 1000 F. Contents of cobalt and/or molybdenum below about 8.5% and 4.6%, respectively, will result in lower strength at at elevated temperatures. Raising either of these elements above about 9.5% and 5.2%, respectively, results in lower toughness at or around room temperature. Raising the amounts of titanium and/ or aluminum and/ or vanadium above the required interrelated ranges lowers toughness and insufiicient amounts of these elements markedly reduces both short time and long time strength at 1000 F. Employing amounts of carbon above about 0.03% or even 0.02% results in loss of low temperature toughness. Additions of tungsten, beryllium and phosphorus can be advantageous in the alloys of the present invention to increase high temperature strength. Columbium and tantalum can be employed to replace all or part of the vanadium when vanadium is present. inch replacement should be made on an atom for atom asis.

Alloys of the present invention can be modified to contain up to about 5% chromium with the chromium replacing nickel weight for weight. By replacing part of the nickel with about 5% chromium, to creep resistance of the thus modified alloys at 1000 F. can be improved. Thus, an alloy containing 5% chromium which was annealed at 1800 F., transformed, age hardened at 900 F. for 3 hours and air cooled, exhibited a life to rupture. at 1000 F. under a stress of 100 'k.s.i. of 263 hours with 30% elongation and 70% reduction in area. However, the yield strength and ultimate tensile strength of this alloy at 1000 F. is significantly lower than the yield strength and ultimate tensile strength of chromium-free alloys of the present invention. In addition, chromium-containing alloys of the kind in question exhibit substantially poorer oxidation resistance than the alloys of the present invention. Thus, in cyclic oxidation tests of alloy specimens in which the criterion of oxidation resistance is weight gain per unit area measured in milligrams per square centimeter mg./cm. the least weight gain being indicative of the best oxidation resistance, alloys of the present invention can be expected to show a weight gain of less than 0.2 mg./cm. after a total of 100 hours at 1000 F. Under the same conditions a similar alloy containing 3% chromium substituted for 3% nickel, exhibits a weight gain of about 0.4 mg./cm. while a well known 5% chromium-1% molybdenum-1% vanadium tool steel exhibits a weight gain of about 0.8 mg./cm. These results show that alloys of the present invention'exhibit at least as good and perhaps better oxidation resistance than alloys currently employed as high-strength alloys at elevated temperatures up to about 1000 F.

When compared to steels outside the present invention, alloys of the present invention exhibit an outstanding combination of room temperature and elevated temperature characteristics making them particularly suitable for structures, parts, components, etc., subjected in use to high and/or prolonged stress at temperatures up to about 1000 F. Specificsteels outside the present invention are set forth in Table IV. It is to be noted with regard to Table IV that, for convenience, iron has been eliminated from the heading. It is to be understood that in each instance, the balance of the alloy is essentially iron together with impurities and small amounts of boron, manganese, silicon, zirconium and/or calcium and other incidental and/or deoxidizing and/or Alloy B, when annealed at 1500 F. prior to aging, exhibited a life to rupture at 1000 F. under a stress of 100 k.s.i. of only 9 hours. Alloy C had low elongation when tested at 1000 F. for tensile characteristics. Alloys N and O are representative of chromium-containing alloys which exhibit impaired oxidation resistance at 1000 F. It is to be observed that low yield strength, low ductility and/or impaired oxidation resistance at 1000 F. limits and/or restricts the commercial use of an alloy at ele vated temperatures. In addition, steels outside the pres-. ent invention can'exhibit low livesto rupture at 1000 F. For example, alloy G, in the condition resulting from annealing at 1500 F., cooling, aging at 900 F. for 3 hours and air cooling, exhibited Shours lifeto rupture at 1000 F. under a stress of 100 k.s.i. This is to be compared to lives of 84 hours and 104 hours ob tained under the same test conditions with aged steels of the present invention annealed at 1800 F. At 300 R, an aged steel of the present invention (alloy No. 2) exhibited a life to rupture of 834 hours at 220 k.s.i. load.

In contrast, a sample of alloy G exhibited a life to rupture of only 38 hours at 800 F. under the much lower stress of 175 k.s.i.

To provide a further contrast between the alloys of r the present invention and known steels, the mechanical characteristics of a commercially available aged steel containing about 18.5% nickel, about 7.5% cobalt, about 4.8% molybdenum, about 0.4% titanium, about 0.1% aluminum and up to about 0.03% carbon are set forth malleableizmg elements. in Table VI.

Table IV C, Ti, Al, Ni, 00, Mo, Cu, W, Ob, Or, V, Alloy Per- Per- Per- Per- Per- Per- Per- Per- Per- Per- Percent cent cent cent cent cent cent cent cent cent cent All of the steels listed in Table IV after being annealed in the temperature range of 1500 F. to 1800 F., transformed by air cooling, aged at 900 F. for 3 hours (that is, age hardened while in the transformed condition) and air cooled, were deficient in one or more of the characteristics necessary to provide commercial utility for service at temperatures up to about 1000 F. Many of the alloys, for example, those listed in Table V, exhibited 0.2% offset yield strengths below about k.s.i. when tested in tensile at 1000 F.

Table VI shows, in comparison with Table 11 set forth hereinbefore, that the alloys of the present invention are much less detrimentally affected by temperatures up to about 1000 F. Thus, at 1000 F. alloy No. 2, an alloy of the present invention, has a 0.2% yield strength of 186 l .s.i., whereas the commercially available steel exhibited a 0.2% yield strength at 1000 F. of only about 129 k.s.i. In addition, the alloy of the present invention maintains a high degree of ductility and toughness in combination with high yield strength at elevated temperatures up to about 1000 F.

The alloys of the present invention are particularly "2? applicable for use as hot work dies and. tooling, for example, hot forging dies and hot extrusion dies, missile and aircraft components heated to temperatures up to about 1000" F. during service, for example, skirt parts of rockets, shafting and rotors for enerating equipment, mortar tubes, gun barrels, chemical plant equipment, etc. The alloy canalso be used for fasteners and other, har

areas-e ware parts intermittently subjected to elevated tempera- Although the present invention has been described in I conjunction with preferred embodiments, it is to beunderstood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A steel having an advantageous combination of mechanical characteristics including strength and toughness'at temperatures ranging from about room temperature up to about 1000" F. and consisting essentially, in percent by weight, of about 14% to about 16% nickel, about'8.5% to about 9.5% cobalt, about 4.6% to about 5.2% molybdenum, about 041% to about 0.8% titanium, about 0.1% to about 1.2% aluminum, up to about 3% vanadium, up to about 0.03% carbon, up to about 0.2% manganese, up to about 0.2% silicon, up to about 0.01% boron, up to about 0.1% zirconium, up to about 0.05% calcium with the balance being essentially iron, the titanium, aluminum and vanadium contents of said steel being so correlated so that the number X calculated by is about 4.5 to about 7.

2. A steel as in claim '1 which contains about 0.5% to about 0.9% aluminum and about 0.6% to about 0.8% titanium.

3. A steel as in claim 1 which is annealed at about 1800 F. for about 1 hour.

4. An annealed and aged steel having an advantageous combination of mechanical characteristics including strength and toughness at temperatures ranging from about room temperature up to about 1000 F. and consisting essentially, in percent by weight, ofabout 14% to about 16% nickel, about 8.5% to about 9.5% cobalt, about 4.6% to about 5.2% molybdenum, about 0.1% to about 0.8% titanium, about 0.1% to about 1.2% aluminum, up to about 3% vanadium, up to 0.03% carbon, up to about 0.2% manganese, up toabout 0.2% silicon, up to about 0.01% boron, up to about 0.1% zirconium, up to about 0.05% calcium with the balance being essentially iron, the titanium, aluminum and vanadium contents of said steel being so correlated so that the number X calculated by V is about 4.5 to about 7.

5. An aged steel as in claim 4 which contains about 0.5% to about 0.9% aluminum and about 0.6% to about 0.8% titanium.

'6. A process for producing a high strength structure adapted to be employed at temperatures up to about 1000" F. comprising providing a steel consisting essentially, in percent by Weight, of about 14% to about 16% nickel, about 8.5% to about 9.5% cobalt, about 4.6% to about 5.2% molybdenum, about 0.1% to about 0.8% titanium, about 0.1% to about 1.2% aluminum, upto about 3% vanadium, up to about 0.03% carbon, up to about 0.2% manganese, up to about 0.2% silicon, up to about 0.01% boron, up to about 0. 1% zirconium, up to about 0.05% calcium with the balance being essentially iron, the titanium, aluminum and vanadium contents of said steel being so correlated so that the number X calculated by X=5 %Ti-|-3 %Al+%V is about 4.5 to about 7, hot forming said steel to structural shape while annealing said steel, cooling said formed steel to effect a transformation therein and thereafter age hardening said formed and transformed steel by heating for about 1 to about 10 hours at about 800 F. to about 1000 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,048,164 Pilling et a1 July 21, 1936 2,712,498 Gresham et al July 5, 1955 FOREIGN PATENTS 409,355 Great Britain Apr. 30, 1934 

1. A STEEL HAVING AN ADVANTAGEOUS COMBINATION OF MECHANICAL CHARACTERISTICS INCLUDING STRENGTH AND TOUGHNESS AT TEMPERATURES RANGING FROM ABOUT ROOM TEMPERATURE UP TO ABOUT 1000*F. AND CONSISTING ESSENTIALLY, IN PERCENT BY WEIGHT, OF ABOUT 14% TO ABOUT 16% NICKEL, ABOUT 8.5% TO ABOUT 9.5% COBALT, ABOUT 4.6% TO ABOUT 5.2% MOLYBDENUM, ABOUT 0.1% TO ABOUT 0.8% TITANIUM, ABOUT 0.1% TO ABOUT 1.2% ALUMINUM, UP TO ABOUT 3% VANADIUM, UP TO ABOUT 0.03% CARBON, UP TO ABOUT 0.2% MANGANESE, UP TO ABOUT 0.2% SILICON, UP TO ABOUT 0.01% BORON, UP TO ABOUT 0.1% ZIRCONIUM, UP TO ABOUT 0.05% CALCIUM WITH THE BALANCE BEING ESSENTIALLY IRON, THE TITANIUM, ALUMINUM AND VANADIUM CONTENTS OF SAID STEEL BEING SO CORRELATED SO THAT THE NUMBER X CALCULATED BY 